The resistor values are not critical, any resistor value between 4k7 and 22K can be used for the two resistors. The freewheeling diode associated with the relay safeguards the transistor from back EMF voltage spikes generated by the relay coil. When it's a PNP transistor such as a BC557, the base terminal of the transistor is coupled with the collector terminal of the optocoupler's internal transistor, the emitter is connected with the positive line and the collector pin is configured with the relay.
#OPTOISOLATOR CIRCUIT DRIVER#
As shown in the following circuit diagrams, the relay driver may consist a NPN transistor or a PNP transistor.Īn external transistor is recommended in a situation where the relay coil resistance is low, below 300 ohms and the relay requires a higher amount of current above 50 mA.Īs can be seen diagram below, a PNP relay driver is connected with the optocoupler. The following concepts show how a relay driver can be configured with an optocoupler using transistors. This is because, most optocouplers cannot handle more than 50 mA current as the load current, therefore the relay coil must have a relatively high resistance so that it does not pass more than 30 or 40 mA current.
So if you want to use the above configuration and connect the relay directly with the optocoupler, then you have to first measure the coil resistance of the relay and make sure it is higher than 300 ohm. Remember, although the above connection diagram looks simple and easy, you must ensure that the relay coil resistance is not below 300 ohms, otherwise the optocoupler may heat up and get destroyed. In the following circuit diagram we can see how a relay can be connected directly connected with the collector of the optocoupler's internal transistor. Connecting Relay Directly with an Optocoupler The photo-transistor output is normally used for driving the preceding isolated stage, for example a relay driver stage. The above action illuminates the internal LED whose light is detected by the photo-transistor causing it to conduct across its relevant pin outs. The idea of operating a relay with an optocoupler is simple, it's all about providing an input DC from the source which needs to be isolated to the LED pin outs via a limiting resistor (as we normally do with usual LEDs) and to switch the photo transistor in response to the applied input triggers. The LED is terminated over a couple of pin outs, while the three terminals of the photo-transistor is terminated over the other three assigned pin outs. An opto-coupler is a device which encapsules an LED and a photo-transistor inside a hermetically sealed, water proof, light proof package in the form of an 8 pin IC (resembling a 555 IC).
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